There are many methods we can use to come upon satisfying note choices when improvising over a predetermined set of chord changes (i.e. a tune). One common way to approach note choice is using chord/scale theory, equating every chord to a scale.
For example: A minor 7 = A dorian. It’s this approach that has people practicing all of their modes!
Certainly the chord/scale approach can help us navigate a tune. Improvising on an A Dorian Scale while an A Minor 7 chords plays will generally produce notes that sound “good” to most folks. However, when an improviser is thinking from chord to chord, and thus mode to mode, the result can often be a solo which sounds disjunct, fragmented or “boxy”, rather than melodic. One way to help us think more linearly, rather than vertically, is to mentally group chords together into key centers.
For example, let’s look at the simple chord changes below and think of them in two different ways. First using chord/scale theory:
Thinking in this way has the advantage that we can choose from several scales for each chord. (Even though I have written only the most common above). In the example above, for example, we might decide to play either a major scale or a lydian scale over the Bb Major harmony. This can help us create interesting tensions as we play over the changes. However, it can also lead to disjointed ideas as we try to keep up with the changing scale tonalities, especially at fast tempi. In such cases, it can help to group the chords into key centers.
If we recognize that all of the chords in the example above are diatonic to the key of Bb major (i.e. they are native to that key and can be created without accidentals) then we realize that we can use the Bb major scale as a basis for our improvisation for all four bars. Let’s look at the chord progression through slightly different eyes:
Thinking in this way greatly reduces the amount of theoretical material we have to contemplate and instrumentally navigate. It also seems to have the added effect of pushing the player to produce more linear and melodic ideas. One pitfall to avoid with this approach, however, is completely disregarding the chord progression as we noodle around in Bb major. This can make our solo sound out of touch harmonically, not to mention meandering and directionless.
Therefore, I suggest using the defining notes of each chord (i.e. guide tones; usually the 3rd and or 6th/7th) as “goal notes” or places of arrival. For example, you may only using the Bb major scale throughout, but you want to avoid landing on a Bb note when the F7 chord is playing. F, A, C or D would be better choices.
Most solos sound better when the player is thinking linearly and melodically. Thinking in key centers is one way to enable a you to do so.